For those who have questioned whether a valveless pulsejet can be successfully designed with the intake attached to the tailpipe alone, the answer is: yes. Today for the first time, I got continuous sustained operation from the ancient and honorable Fo Mi Chin. The engine started easily and sustained on propane fed at the center of the intake flare through 1/8-inch copper tubing. I was able to get two runs of 3-4 minutes in order to get still shots of the hot engine. This was at about 40 degF, and it's obvious to me that the device would have sustained operation indefinitely. Altogether I got about five good runs before my battery ran down and I was unable to get spark for starting.

The bad news is that this success was attained at an overall engine length of 47 [count 'em - FORTY-SEVEN] inches! That's the longest length needed for any of my designs. I think this is because the intake layout of Fo Mi Chin is very poor in terms of acoustic resonance. Naturally, the frequency was about half that of the "standard" FWE, possibly even less.

FUEL and AIR SETUP -

Propane vapor was set up at 12 PSIG on the regulator to the needle valve, then fed through 6 ft of rubber hose and about 18 inches of 1/8-inch copper tube wound around the tailpipe and aimed into the intake flare at dead center, straight in. Starting was secured with almost a trickle of fuel and the open end of the shop vac hose about 10-12 inches back from the flare and slightly to one side [high-speed nozzle not used]. Once sustaining, a wide range of fuel flows could be used to keep the engine running, though I was never able to set it too rich to run [probably due to the restriction of the small copper tubing].

I have to agree with Steve, Eric et al that for these rear-facing intakes that's almost the ideal fueling spot. There was absolutely no flame visible from the tail or the intake flare during sustained running, although there was a lot of flame during starting [pops and bangs], especially when the overall length was just wrong.

LENGTHS -

I can try only a few lengths, by swapping out sections of tubing, i.e. I cannot experiment with fine increments. Lengths of 33 and 37 inches would produce good roars with air, but never would sustain. One experiment with the ludicrous overall length of 61 inches gave me one brief sustained run that gradually weakened and died over about ten seconds, and I interpreted that as being too long. The next thing I tried was coupling two sections to the tailpipe for an overall length of 47 inches, which must be nearly perfect, for it started easily and caught hold running immediately. So, that was basically a three-piece tailpipe with two fairly leaky couplings - the end section did happen to have a decent flare, however.

The length ratios will be interesting to some who followed the 'Hinote Criteria' related threads. The overall length L of 47 inches = 1194 mm. The total length of the bent intake from flare to spout is about 122 mm, only .10L - only half the size dictated by the criteria! The 'Reynst Point' - the station at which the intake air spouts into the engine - is almost at the desired L/8: 163 mm from the apex of the front dome. The actual value is about .14L. This coupled with the short intake length puts the flare at a theoretically pitiful wave path length of 285 mm from the apex = .24L, far short of the "ideal" L/3 point - yet the engine runs! All this suggests that the 'Reynst Point' might be the most critical of all the criteria [but, maybe for poorly proportioned engines like this, there are other factors involved and this is just coincidence - who knows?]. Anyway, the point is that as far as the criteria are concerned, only the Reynst Point comes close, on this engine build.

HEAT -

The chamber heat is unusual for me in that it seems astonishingly uniform all over, including the front dome. I just couldn't see much variation. Perhaps when I get the photos processed some detail will show up more prominently. There was no observable heating of the tailpipe, but I was using the Bukowsky Quik-Mount [i.e. vise] right behind the nozzle point, so there was a lot of heat sinking right there. The copper eventually became discolored where it wrapped around the tailpipe, aft of the vise jaws. My fancy white paint on the dome and the nozzle zone, smoked vehemently for a while and basically turned to chalk-like powder with no lasting discoloration [probably Titanium oxide].

The strangest heat observation came from looking into the tailpipe from a few feet back. There was the usual lovely purple that comes from seeing red hot steel through a slug of blue flame, but the INSIDE surface of the intake spout [way back at the front end of the tailpipe] was BRIGHT orange, while the outside surface visible just above it was relatively dark!!! [Recall that the spout is cut back at a raked angle so a little of the interior is visible from directly behind.] I swear I am not making this up. One of the strangest things I've ever seen. Note that the spark plug on this engine is mounted low on the front dome, and is not visible through the tailpipe.

That's about it. All comments welcome. Pictures, maybe next week - I have half a roll of film to work through before they can be processed.

I realized last night after I wrote that post, that the reason the "inside" of the intake tube is hot is because it's the inside of the front of the tube, where the exhaust gas collides with it, and the "outside" I can see is the rear wall of the tube, where the exhaust stream goes past and around it.

I don't know why that didn't occur to me immediately, except that it's just so visually striking when you see it - the only outside part you can see is cool, while the interior part you can see appears uniformly bright orange.

It makes me wonder how that structure would hold up if it were stainless instead of mild steel. I know they used Inconel for the intruding intake spout in the NRL engine, and fear of rapid erosion by oxidation must have been the reason for that!

Mike Everman wrote:Larry, very cool. never have I more wanted you to get a cheap digital camera!
.1L, whoa. Any guess on thrust?

No idea on thrust, though there was a pretty good stream of hot wind coming out the rear. I could put my hand within about eight inches from it before it got too unpleasant to be there. I could get much closer to the intake, and I'm sure you wouldn't see any flame there, even in total darkness.

Yes, the .1L is really interesting - I actually expected the location of the flare to be the dominant factor, but that doesn't seem to be the case. Of course, I think the engine would probably be in much better tune if the intake were lengthened so that it hits the L/3 point, but satisfying the 'Reynst Point' seems to be the real key. Of course, this is from a very limited trial, and may be too quick a conclusion. But it's certainly striking that such a short intake pipe can work in sync with such a long engine.

Mike Everman wrote:I am starting to think that something important to intakes is L/D being something like 6:1, then tail to match. I'll be testing this tonight on my rev10.

That's interesting. Just so everyone will know, the intake on Fo Mi Chin is 1/2-inch EMT, really over 5/8-inch ID , and the tailpipe is 1.25 inch OD but pretty thin-walled, so probably 30 mm ID or thereabouts.

What I'm wondering is whether the intake can just become a non-resonant [or at least non-synchronous] element if it's radically "too short" - a sort of tubular version of the Reynst Pot annular gap. That's what I had in mind with the Reynstodyne Shark(TM) engine, thinking nothing about any kind of intake stack resonance at the time. It makes me wonder if just lengthening the tail [suggested by ben Brockert when he saw it] and shortening the front end, so the port where the stack joins the pipe would be at L/8, might bring the Shark into sustaining operation while retaining the compact size of the whole.

Kind of like Mark's logan. It's intake is very short too. I've been wondering if an intake could be a series of holes in 3/8-1/2" thick material, so, as you say, it has no resonant properties of note. still it would have a radius type inlet flare on each, but sharp on the inside.

Mike Everman wrote:Kind of like Mark's logan. It's intake is very short too. I've been wondering if an intake could be a series of holes in 3/8-1/2" thick material, so, as you say, it has no resonant properties of note. still it would have a radius type inlet flare on each, but sharp on the inside.

Ha ha! I visualized the same thing about an hour ago while driving back home - an FWE with a ring of little holes all around the L/8 station, with each one countersunk slightly to form a set of "micro nozzles" going in! The only problem I could see is that the scheme doesn't provide anything to get the air up to the front where it's supposed to be for the explosion. I guess you could provide a conical ring just inside to act as a turning vane, if you thought it was needed.

Another minor detail of note is providing fuel so that it mixes properly. How about a manifold ring with a bunch of little capillary tubes feeding into the holes?

I was actually thinking about the "shark" the other day. I was pretty sure that you could get it running on acetlyne (did you ever try that? I can't remember) I don't think you will need to shorten the CC, but the tailpipe definately looks too short for an engine that size. Perhaps squashing the tailpipe down a bit would help as well?

I like punched in holes in the sheet material, the little volcano shape will plug even better in exhaust phase. they'll be easy to plug too, to find the right number, so you should have more than you need.
OH! I KNOW! Pick me! (hand in air) ;-) If you make the cc very close double walls and punch through both, you can feed fuel between and it will preheat/vaporize liquid, and EACH hole will have fuel injecting around the "lips of the volcanos".

If you made the holes very small you could "sweat" in the fuel. Or if you have ever sliced your finger so that only a few layers of skin are cut away, you will notice how the entire surface area bleeds, but slowly.
I wonder after my jam jar malady, the thickness of the jam jar lid becomes apparent as to whether it works or not. Jam jars do not like thick lids. Perhaps the super-ported pulsejet should also have thin walled turbulence-inducing ports, or maybe Mike's plate tectonics and volcanism structures would work too. He's always raising his hand, that Mike.
Mark

Finally - shots of my successful test run. Note the two extensions that brought the overall engine length to 47 inches. Note also the unusually even heating of the chamber [heating of the front dome is heavily masked by the chalky remnants of white lacquer].